[C++] Methods to calculate the length of an SBE message

[agmt]

Implements: #291

[lgtm-com]

This pull request introduces 1 alert when merging 693c052 into 6ec0439 - view on LGTM.com

new alerts:

• 1 for Boxed variable is never null

[mjpt777]

Will look in more detail in at a later date. Got a bit too much on at the minute.

One thing on naming. Something that will calculate the total so be named to reflect that it is not a simple accessor. Something like computeEncodedLength.

[agmt]

@mjpt777 noticed. New function names are: static computeLength(...) (to predict message length before encoding) and decodeLength() const (to find length in the beginning of decoding).

[mjpt777]

I see you were fixing some warnings in addition to this. I've picked those up independently.

[agmt]

I've rebased on master

[mjpt777]

Be good to get @tmontgomery view on this.